1. Field of the Invention
The present invention relates to an imaging lens assembly and more particularly to an imaging lens assembly has triplet lens elements and used in portable devices.
2. Description of the Prior Art
In recent years, the development of the optical lens assembly in digital cameras, web cameras and mobile phone cameras becomes more rapid with the sharp increase in demand of people for high image capturing quality. High resolution, large field of view for sub camera, smaller size and etc are all considered more and more by consumers.
Generally, for the sake of meeting customers' demands, especially demands for high imaging quality, a conventional lens assembly is designed to be a three-lens structure comprising a first glass lens element with positive refractive power, a second glass or plastic lens element with negative refractive power and a third lens element with non-spherical surfaces, whereby to reduce aberrations and the total track length of the optical system. However, the manufacturer found it hard to maintain a high yield of lens assembly while keeping a relative high imaging quality since the cementing procedure conducted on the first and second lens elements is difficult to control and the aspheric coefficients of lens elements are not preferable yet. Besides, the defocus state of a lens assembly will significantly influence the performance of the whole camera in auto-focusing which was confirmed by the assembling factory. And the aspheric coefficient of lens element does play an important role in confining the defocus state.
How to achieve a quick focusing module with less aberrations, and balance the weight between high imaging quality and manufacturing feasibility is the key topics in lens assembly fabrication, therefore, there is need to construct a favorable lens assembly structure with a good adjustment of its structure and aspheric coefficients.
A patent CN2824074 discloses an imaging lens assembly which comprises, in order from the object-side to the image-side, a first lens with positive refractive power has a convex object-side surface, a second lens with refractive power shrinking along the optical axis direction has a non-spherical object-side surface, a third lens with negative refractive power has a non-spherical object-side surface being convex. And both the second and third lenses are made of plastic. This imaging lens assembly can achieve good imaging quality when keep a relative short total track length since the second lens with negative refractive power contributing to a thin central thickness. This invention also can reduce the chromatic aberration and improve the color saturation of the image. But the invention is not conducted through adjusting relationships between aspheric coefficients, not performed with a good defocus state either.